Types of ophthalmologic apparatuses include ophthalmologic imaging apparatuses for obtaining images of an eye and ophthalmologic measuring apparatuses for measuring characteristics of an eye.
Examples of ophthalmologic imaging apparatuses include an optical coherence tomography (OCT) apparatus that obtains cross sectional images using OCT, a retinal camera that photographs a fundus, a Scanning Laser Ophthalmoscope (SLO) that obtains images of a fundus by laser scanning with a confocal optical system, a slit lamp that obtains images by photographing an optical section of a cornea using slit light, etc.
Moreover, examples of ophthalmologic measuring apparatuses include an eye refractivity examination apparatus (refractometer, keratometer) that measures refractive properties of an eye, a tonometer, a specular microscope that obtains properties of a cornea (cornea thickness, cellular distribution, etc.), a wave-front analyzer that obtains aberration information of an eye using a Shack-Hartmann sensor, etc.
Regarding ophthalmic examinations using these apparatuses, in terms of precision and accuracy of examinations, position matching between the optical system of the apparatus and an eye is very important. Types of the position matching include alignment and tracking. Alignment is an operation to move the optical system of the apparatus to a prescribed position for eye examination. Tracking is an operation performed after alignment for maintaining the positional relationship achieved by the alignment by detecting the movement of the eye and causing the optical system of the apparatus to follow this eye movement.
Further, the position matching includes the position matching in the direction along the optical axis of the optical system of the apparatus (the position matching in the axial direction (z-direction)) and the position matching in the direction perpendicular to the optical axis (the position matching in the xy-direction)). Alignment includes xy alignment for matching the optical axis of the optical system of the apparatus with the eye axis and z alignment for positioning the optical system of the apparatus in the position prescribed distance away from the eye. It should be noted that the prescribed distance applied in z alignment is a preset value and called the working distance.